core: Refactor zotonic-modules script

Changes made include:
* Improved bin/zmm usage/help text
* Coding style now conforms to the Python Style Guide
* Removed the test fixture
* Add Arjan Scherpenisse, the original author of the script, to the authors list
* Minor typo corrections
mod_admin_modules: Display 'actions' menu for site-specific modules
mod_admin_modules: Catch 'case' clause error
core: Document code and minor refactoring